Hi there..When I have my obd2 sensor up and running and I am monitoring my speed it matches up with my speedometer gauage on my truck. But when i use a gps android program I am roughly out about 4-5kms. So I am confused as to which one I should be going by. I have stock tires on my truck as well.

Hi

To get an accurate GPS speed reading, you need to be driving in a straight line, at a constant speed. GPS will not handle changes of direction and speed changes very well, so make sure you’re on a straight road at a constant speed before you read the GPS speed off, for the most accurate speed reading via GPS

Your speed reading on OBD/speedos can be affected by:

* Wheel diameter
* Tyre pressure (though not so much)
* Calibration of the speedometer
* Calibration of the ECU via the wheel speed sensor (usually ABS reluctor rings can be used for speed readings)

Usually, speedos are clocked slightly faster than the actual vehicle speed and it is normal to see a lower reading via OBD in most cases (dependant on the manufacturer). Mainly due to liability (etc) reasons.

Torque provides a GPS vs OBD speed reading widget for the purpose of knowing how far apart your speedometer and OBDs speed readings are – for 4/5km/h out I would not be overly concerned